The present invention relates to a power transfer system for controlling the distribution of drive torque between the front and rear wheels of a four-wheel drive vehicle.
Due to increased consumer demand for four-wheel drive vehicles, a plethora of different power transfer systems are currently utilized for directing power (i.e., drive torque) to all four wheels of the vehicle. For example, in many "part-time" power transfer systems, a transfer case is incorporated into the driveline and is normally operable in a two-wheel drive mode for delivering drive torque to the driven wheels. When four-wheel drive is desired, a mode shift mechanism can be selectively actuated by the vehicle operator for directly coupling the non-driven wheels to the driven wheels for establishing a part-time four-wheel drive mode. As will be appreciated, motor vehicles equipped with such a part-time power transfer systems offer the vehicle operator the option of selectively shifting between the two-wheel drive mode during normal road conditions and the part-time four-wheel drive mode for operation under adverse road conditions. An example of a part-time transfer case is disclosed in commonly-owned U.S. Pat. No. 4,770,280 to Frost.
Alternatively, it is known to use "on-demand" power transfer systems for automatically directing power to the non-driven wheels, without any input or action on the part of the vehicle operator, when traction is lost at the driven wheels. Modernly, the on-demand feature is incorporated into the transfer case by replacing the mode shift mechanism with a clutch assembly that is interactively associated with an electronic control system and a sensor arrangement. During normal road conditions, the clutch assembly is maintained in a non-actuated condition such that drive torque is only delivered to the driven wheels. However, when the sensors detect a low traction condition at the driven wheels, the clutch assembly is automatically actuated to deliver drive torque on-demand to the non-driven wheels. Moreover, the amount of drive torque transferred through the clutch assembly to the non-driven wheels can be varied as a function of specific vehicle dynamics, as detected by the sensor arrangement. One example of such an "on-demand" power transfer system is disclosed in commonly-owned U.S. Pat. No. 5,323,871 to Wilson et al wherein the electronically-controlled clutch assembly is operable for automatically controlling the amount of drive torque transferred to the non-driven wheels as a function of the wheel speed difference (i.e., the wheel slip) between the driven and non-driven wheels.
As a further alternative, some vehicles are equipped with full-time power transfer system having a transfer case equipped with a center differential that functions to permit interaxle speed differentiation while transferring drive torque to both of the front and rear drivelines. To minimize loss of traction due to wheel slippage, full-time transfer cases are typically equipped with a slip limiting device for locking the center differential to prevent speed differentiation and, in effect, establishing a part-time four-wheel drive mode. Examples of manually-actuated differential lock-up arrangements are disclosed in commonly-owned U.S. Pat. No. 3,848,691 to Dolan and U.S. Pat. No. 4,677,873 to Eastman. An automatic differential lock-up arrangement is disclosed in commonly-owned U.S. Pat. No. 3,845,671 to Sharp et al. wherein an electrically-controlled clutch assembly is actuated to lock-up the center differential when speed differentiation due to a wheel slip condition is detected as exceeding a predetermined value. In addition, torque-biasing differential lock-up arrangements are disclosed in commonly-owned U.S. Pat. No. 4,031,780 to Dolan et al. and U.S. Pat. No. 5,046,998 to Frost, which both utilize a viscous coupling to progressively modify the torque distribution in proportion to the magnitude of the speed differentiation across the center differential. Finally, electronically-controlled full-time transfer cases are disclosed in U.S. Pat. No. 4,718,303 to Fogelberg and U.S. Pat. No. 4,860,612 to Dick et al. wherein an electromagnetic biasing clutch is provided across the center differential to controllably bias the torque delivered to the front and rear drivelines in response to wheel slip.
To accommodate differing road surfaces and conditions, many of the above-referenced transfer cases are equipped with a gear reduction unit for providing high-range (i.e., direct drive) and low-range (i.e., reduced ratio drive) speed ratios in conjunction with the various four-wheel drive modes. Most commonly, the gear reduction units used in such dual-speed transfer cases include either a layshaft arrangement or a planetary gear assembly. However, in most current four-wheel drive vehicles, the transfer case can only be shifted between the four-wheel low-range drive mode and the four-wheel high-range drive mode when the motor vehicle is in a substantially non-motive condition. Unfortunately, the need to stop the vehicle prior to shifting between the available four-wheel drive speed ranges is inconvenient, particularly upon encountering road conditions or surface terrain where continuation of the vehicle's rolling momentum would assist in overcoming the conditions encountered. As such, gear reduction units have been designed which permit the vehicle operator to shift "on-the-fly" from the four-wheel low-range drive mode into the four-wheel high-range drive mode without stopping the vehicle. For example, U.S. Pat. No. 5,054,335 to Andrews discloses a transfer case having a synchronized shift arrangement for a layshaft-type gear reduction unit while commonly-owned U.S. Pat. No. 5,346,442 to Eastman discloses a transfer case having a synchronized shift arrangement for a planetary-type gear reduction unit. However, while both designs advance the art, the need still exists to develop a gear reduction unit that can also be shifted "on-the-fly" from the four-wheel high-range drive mode into the four-wheel low-range drive mode.